Biogeographic gradients of picoplankton diversity indicate increasing dominance of prokaryotes in warmer Arctic fjords

Climate change is opening the Arctic Ocean to increasing human impact and ecosystem changes. Arctic fjords, the region’s most productive ecosystems, are sustained by a diverse microbial community at the base of the food web. Here we show that Arctic fjords become more prokaryotic in the picoplankton (0.2–3 µm) with increasing water temperatures. Across 21 fjords, we found that Arctic fjords had proportionally more trophically diverse (autotrophic, mixotrophic, and heterotrophic) picoeukaryotes, while subarctic and temperate fjords had relatively more diverse prokaryotic trophic groups. Modeled oceanographic connectivity between fjords suggested that transport alone would create a smooth gradient in beta diversity largely following the North Atlantic Current and East Greenland Current. Deviations from this suggested that picoeukaryotes had some strong regional patterns in beta diversity that reduced the effect of oceanographic connectivity, while prokaryotes were mainly stopped in their dispersal if strong temperature differences between sites were present. Fjords located in high Arctic regions also generally had very low prokaryotic alpha diversity. Ultimately, warming of Arctic fjords could induce a fundamental shift from more trophic diverse eukaryotic- to prokaryotic-dominated communities, with profound implications for Arctic ecosystem dynamics including their productivity patterns.

Seawater samples were collected during the HE533 expedition between 20.05.2019 and 06.06.2019.Water samples were collected in triplicates (A-C) with Niskin bottles mounted on a Seabird' SBE911+' CTD probe with additional turbidity, oxygen, and fluorescence sensors.
A total of 20 L of seawater at three different depths (3m, DCM which varied between 8-28m, and 40m depth) was pooled and gravity-filtered through 200 µm and 20 µm mesh size sieves, and subsequently filtered through 3 µm and 0.2 µm polycarbonate filters (147 mm diameter, Millipore) using a Millipore Tripod filterholder and a peristaltic pump within a time window of max.30 min.Warm lysis buffer was added to filters, followed by snap freezing and storage at -80°C until further processing.Picoplankton DNA was extracted using the NucleoSpin® Soil kit (Macherey-Nagel, Germany) following the manufacturer's protocol.For the sample lysis step, a bead beater was used to break up the cells (MagNA Lyser, Roche).As other datasets in this manuscript did not have replicates, we subsampled only the first replicate (A) of the dataset for further analysis.Additionally, we excluded clear outliers from analysis, which were due to a Crysocromulina spp.bloom during the HE533 expedition.

DNA:
To extract DNA from the 0.2 µm polycarbonate filters, the Genomic DNA from soil (NucleoSpin® Soil) kit was used following the manufacturer's protocol with a minor modification: the sample lysis was conducted with a bead beater (MagNA Lyser, Roche) for 2 × 30 seconds at 55,000 rpm.The variable region 4 (V4) of the small subunit ribosomal RNA gene (16S for prokaryotes and 18S for eukaryotes) was used as a molecular marker to determine the taxonomic community composition.Primers were selected in accordance with the Earth Microbiome Project (http://www.earthmicrobiome.org/protocols-and-standards/)using prokaryotic primers (515F -806R) 1 and eukaryotic primers (TA-Reuk454FWD1 -TAReukREV3) 2 with overhanging Illumina adapters.The library preparation preceding the sequencing followed standard protocols (16S Metagenomic Sequencing Library Preparation, Illumina, Part #15044223 Rev.B; Illumina Technology).The amplicon libraries were paired-end sequenced on the Illumina MiSeq sequencing platform.The prokaryotic samples were sequenced at the Alfred Wegener Institute in Bremerhaven, Germany, and the eukaryotic samples were sequenced at the Leibniz Institute on Aging (FLI) in Jena, Germany, using 300-bp paired-end sequencing on a MiSeq Sequencer (Illumina) with a MiSeq Reagent Kit v3 (600-cycle).All samples were demultiplexed using bcl2fastq (Illumina) with barcode mismatches set to 1.
Figure S1.Sample locations in northern Norway.Individual fjord names are indicated in the map.Map created with Ocean Data View 3 .

Figure S3 .
Figure S3.Sample locations in Svalbard.Individual fjord names are indicated in the map.Map created with Ocean Data View 3

Figure S6 .
Figure S6.Sample locations in West Greenland.Individual fjord names are indicated in the map.Map created with Ocean Data View 3 .

Figure S7 .
Figure S7.Temperature -salinity plot of all sites from surface water samples.Sites are colorcoded according to geographic region.

Figure S15 .
Figure S15.Relative abundances of prokaryotic (a) and picoeukaryotic (b) taxa at the Order level.

Figure S16 .
Figure S16.ASV abundance of prokaryotes annotated as "autotroph" based on literature research mapped against functional inference derived from PICRUST2 including all KOs involved in autotrophy (see TableS4for details).

Figure S17 .
Figure S17.Relative proportion of bacteria, archaea and eukaryotes in each station separated between Arctic, subarctic and temperate regions.

Figure S18 .
Figure S18.Microbial beta diversity distance (Aitchinson distance) based on 16S rRNA (prokaryotes) and 18S rRNA (eukaryotes) sequences analyses against hydrodynamic distance based on the inverse of the normalized log10 synthetic particle concentration.

Figure S19 .
Figure S19.Total number of numerical synthetic drifters per site and for individual temporal bins.

Table S1 .
Tested explanatory variables for prokaryotic and eukaryotic beta diversity distribution.

Table S3 .
Significance levels of alpha diversity measures (Richness and Pielou Evenness) between bioclimatic subzones using two-sample t-test with Bonferroni adjustment of pvalues.

Table S4 .
Significance levels of differential abundance of functional trophic groups between bioclimatic subzones using two-sample t-test with Bonferroni adjustment of p-values.

Table S5 .
Cross-links to dataset publications and data availability.